Electrical vehicles or photovoltaic power generation systems use an electric power conversion apparatus represented by an inverter. In order to improve efficiency of the entire system, it is necessary to reduce the power loss through the electric power conversion apparatus.
Since about 50% of the loss through the electric power conversion apparatus is generated in the semiconductor switching element, reduction in power loss through the semiconductor switching element is an important technology.
Recently, an insulated gate bipolar transistor (IGBT) or a metal oxide semiconductor field effect transistor (MOSFET) which is a voltage-controlled transistor is widely used as the semiconductor switching element in the electric power conversion apparatus.
Such a switching element is turned on when a voltage of about +15 V is applied between the gate and source (or gate and emitter) from the gate drive circuit so that the gate-to-source (or gate-to-emitter) voltage exceeds a threshold value, and the current flows between the drain and the source (or the collector and emitter).
In order to turn off the switching element, the gate-to-source (or gate-to-emitter) voltage is maintained to be equal to or lower than a threshold value by setting the output voltage of the gate drive circuit to about 0 V or −15 V.
Gate electrodes of the MOSFET and the IGBT are covered by an oxide film. A gate-source capacitance (or gate-emitter capacitance) is formed between the gate electrode and the source electrode (or emitter electrode), and a gate-drain capacitance (or gate-collector capacitance) is formed between the gate electrode and the drain electrode (or collector electrode).
Therefore, an equivalent circuit in the vicinity of the gate of the switching element includes a parallel connection between the gate-source capacitance (or gate-emitter capacitance) and the gate-drain capacitance (or gate-collector capacitance).
Typically, a gate resistance is connected between the gate electrode of the switching element and the gate drive circuit. Therefore, if a voltage of about +15 V is applied from the gate drive circuit to turn on the switching element, the gate-source capacitance (or gate-emitter capacitance) and the gate-drain capacitance (or gate-collector capacitance) are charged through the gate resistance.
In a standard hard-switching circuit, the values of the gate circuit is changed for the purpose of changing the charge time for the aforementioned capacitances, thereby adjusting the switching time of the switching element.
The switching loss can be reduced by shortening the switching time. Therefore, in order to realize the low loss in the switching element, there is proposed a method of reducing the switching time by reducing the value of gate resistance.
In an inverter circuit shown in FIG. 1, switching elements are provided in both high and low sides to constitute an upper arm and a lower arm, and they are controlled such that, when any one of both sides is turned on, the other side is turned off. For example, the switching element in the low side is turned off when the switching element in the high side is turned on. Conversely, the switching element in the low side is turned on when the switching element in the high side is turned off. If the switching elements in both sides are turned on at the same time, a short-circuit current equal to or higher than a rated current of the switching element flows through the upper and lower arms so that the switching element may break down. Therefore, a period of time (dead time) during which both elements are in OFF state at the same time is provided so that not both elements are in ON state at the same time.
Here, as shown in FIG. 2, if one of the switching elements is turned on from the turn-off state, a drain-to-source voltage (or a collector-to-emitter voltage) of the other switching element automatically changes from 0 V to an input DC voltage. At the same time, the gate-drain capacitance (gate-collector capacitance) is charged to a value approximately equal to the input DC voltage.
In this case, a displacement current may flow through the gate-drain (or gate-collector) capacitance to charge the gate-source (or gate-emitter) capacitance so that the gate voltage exceeds a threshold value, and the switching element may be erroneously turned on.
The peak value of the displacement current is proportional to the time variation rate (dV/dt) of the drain-to-source (or collector-to-emitter) voltage of the switching element. Therefore, if the switching time is reduced to reduce switching loss of the switching element, the peak value of the displacement current increases so that the switching element is easily turned on due to the principle described above.
In this regard, there has been a study on a gate drive circuit for preventing the gate voltage from exceeding a threshold value by short-circuiting the semiconductor switching element between the gate and the source (or between the gate and the emitter).
However, in such a circuit, it is necessary to further prepare elements including two diodes, a single resistor, a switching element, and the like in addition to the components in the typical gate drive circuit. Further, since it is necessary to operate the aforementioned switching element in synchronization with a control signal of a typical gate drive circuit, a control device for generating a synchronization signal is also necessary. This makes the design of the entire circuit complicated.